System, assemblies and methods for mechanical-thrust power conversion multifans

ABSTRACT

A multi-rotor system for providing air thrust is disclosed comprising at least one multi-rotor assembly. The multi-rotor assembly comprising at least two rotors rotatable about a common axis wherein the outer radius of a first rotor, is substantially similar to the inner radius of the second rotor. An airborne vehicle is also disclosed that is adapted to perform vertical takeoff and landing (VTOL). The airborne vehicle comprising at least two multi-rotor system disposed substantially symmetrically around the center of gravity of the vehicle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application of PCT InternationalApplication No. PCT/IL2017/050069, International Filing Date Jan. 18,2017, entitled “System, Assemblies and Methods for Mechanical-ThrustPower Conversion Multifans”, published on Jul. 27, 2017 as InternationalPatent Publication No. WO 2017/125923, claiming the benefit of U.S.Provisional Patent Application No. 62/280,762, filed Jan. 20, 2016, allof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Methods and means for providing thrust in fluidic environment such as inthe air are known for many years and include various types ofpropellers, fans, jet engines, rocket engines and the like. Similarly,methods and means for converting flow of fluidic material, such as air,to mechanical/electrical energy are also known for many years andcomprise wind turbines, wave/tide energy conversion systems, and thelike. Means and methods for providing thrust for an object in air, forproviding forward/sideways forces or for providing lift forces, such asin helicopters or similar vertical takeoff and landing (VTOL) vehicles,are also known.

The efficiency of such means and methods may be measured, basically, bythe ratio between the resulting effective thrust energy/power and themechanical/electrical energy/power entered into the converting means.Similarly, in means for converting flow of fluids intomechanical/electrical energy the ratio measuring the efficiency will bebetween the resulting mechanical/electrical energy/power and the fluidsenergy/power given to the conversion means.

Typically, reduction of the efficiency may result from mechanicalfriction, from fluidic turbulences resulting non-consumable energy loss,fluidics energy converted to noise, mechanical vibrations, etc.

Additional efficiency factor may be the efficiency of exploitation ofarea, space and related physical dimensions. For example, the thrustprovided by a propeller or helicopter rotor is different in differentareas of the propeller/rotor. FIGS. 1A-1C schematically presenthelicopter 100 in top and side views and a graph of the helicopter'srotor aerodynamic performance, respectively, as known in the art. Rotor102 of helicopter 100 may be rotated in a rotation speed that is limitedby the tangential speed of the rotor's blade tips, VR_(T), that shouldnot exceed the speed of sound, and practically not exceed 0.85 of thespeed of sound. While the rotor blade tip tangential speed may be closeto, and may not exceed the speed of sound, the tangential speed ofpoints along the rotor blade, vary from zero at the axis of rotation,and increases linearly with the radius towards the rotor's blade tip.The aerodynamic efficiency of rotating blades decreases as their linear(=tangential) speed decreases or as their linear speed approaches thespeed of sound due to the abrupt change in the density of the air. Thearea of rotor 102 may be divided, roughly, into two concentric zones 107and 108. Zone 107 is the external zone, defined between circle 104,depicting the path of the rotor blades' tips and circle 106. Zone 108 isthe internal zone defined inside circle 106. As the tangential speed ofpoints along the rotor's blades reduces, the aerodynamic lift of theblade at such points reduces until, at points along circle 106, the liftof the blade there is smaller than the aerodynamic drag produced at thatpoint, as is depicted in FIG. 1C, where graph 110 describe theaerodynamic lift at points along the blade and graph 112 describes theaerodynamic drag produced by the rotor's rotating blades. It may be seenthat the drag produced by the rotor's blades is substantially constantwith the location along the blade, while the aerodynamic lift dropssharply at points located along circle 106. The efficiency of suchthrust producing means may, thus, be defined as the ration of the areausable for producing lift to the total area exploited by the thrustproducing means. For example, if the radius of circle 106 is ⅔ of theradius of the blade's tip 104, the Rotor area(s) Efficiency ratioRE_(S), will be:

${RE}_{S} = {\frac{\pi\left( {{R\; 1^{2}} - {R\; 2^{2}}} \right)}{\pi\; R\; 1^{2}} = {\frac{{R\; 1^{2}} - \left( {\frac{2}{3}R\; 1} \right)^{2}}{R\; 1^{2}} = {\frac{5}{9} = 0.55}}}$

Generally speaking the thrust obtainable from a rotating rotor isproportional to the square of the linear speed of a given part of therotor's blade, thus the linear speed of the rotor's blades has a largeimpact on the obtainable thrust of a rotor. Further, noise caused by therotation of a rotor or a propeller is mainly due to air shear caused byneighboring/adjacent air flows having different air flow speeds.Therefore obtaining a given total thrust from a rotor system where thedifference in airspeeds of adjacent airflows is lower will reduce thenoise caused by that rotor.

It would be beneficial to improve the rotor area efficiency figure, andeven more beneficial to improve this efficiency factor while improving,or at least not diminishing other aspects of the overall performancefigures, such as lower self-weight, reduced noise losses, etc.Improvement of the efficiency of a rotor/propeller may also reduce thenoise it produces during operation, which mainly results fromnon-laminar flow induced by the rotor/propeller. Thus, reduction ofnon-laminar flow of fluid through the rotor/propeller, by means ofslowing the speed of the blades' tips, by controlling and directing thefluid's flow before and after the rotor/propeller and the like, mayincrease the useful thrust while reducing the produced noise.

SUMMARY OF THE INVENTION

A multi-rotor system for providing air thrust is disclosed comprising atleast one multi-rotor assembly. The multi-rotor assembly comprising atleast two rotors rotatable about a common axis wherein the outer radiusof a first rotor, is substantially similar to the inner radius of thesecond rotor.

According to some embodiments of the invention the multi-rotor systemfurther comprising driving means adapted to rotate each of the rotors ina rotational speed independent of the rotational speed of the otherrotors.

According to some embodiments of the invention the at least two rotorsare disposed and rotatable in a common plane.

According to some embodiments of the invention the multi-rotor systemfurther comprising duct disposed closely around the outer radius of theout-most rotor and directed with its air flow direction coaxially withthe common axis of the multi-rotor system. The multi-rotor system mayfurther comprise at least one additional air duct disposed closelyaround outer radius of one other rotor.

According to yet additional embodiments an airborne vehicle is disclosedthat is adapted to perform vertical takeoff and landing (VTOL). Theairborne vehicle comprising at least two multi-rotor system disposedsubstantially symmetrically around the center of gravity of the vehicle.Each multi-rotor system comprising at least two rotors rotatable about acommon axis wherein the outer radius of a first rotor, is substantiallysimilar to the inner radius of the second rotor, and driving meansadapted to rotate each of the rotors in a rotational speed independentof the rotational speed of the other rotors.

According to embodiments of the invention the airborne VTOL vehicle isadapted to provide vertical thrust when in takeoff and in landingmaneuvering and is further adapted to provide horizontal thrust when inflight maneuvering.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIGS. 1A-1C schematically present an helicopter in top and side viewsand a graph of the helicopter's rotor aerodynamic performance,respectively;

FIGS. 2A and 2B schematically depict a coaxial multi-rotor system and agraph of the rotors' blades velocities of this system, respectively,according to embodiments of the present invention;

FIG. 3 schematically presents cross section through a coaxialmulti-rotor system, showing only the left side of the cross section,which is symmetric about a symmetry line, according to embodiment of thepresent invention;

FIG. 4 schematically presents coaxial multi-rotor system according toembodiments of the present invention;

FIG. 5 schematically presents lift system comprising four coaxialmulti-rotor systems, each comprising a plurality of coaxial rotors,according to embodiments of the present invention;

FIG. 6 is a schematic cross section in coaxial multi-rotor systemaccording to embodiments of the present invention;

FIGS. 7A-7C are schematic illustrations of a coaxial multi-rotors systemin top view, in first cross section view and in a second cross sectionview, respectively, according to embodiments of the present invention;

FIGS. 8A-8D schematically present various mechanical arrangements ofpowering multi-rotor systems using electric motors, according toembodiments of the present invention;

FIGS. 9A and 9B are schematic cross section of an isometric view and afront view of a cross section, respectively, of a multi-rotor systemaccording to embodiments of the present invention; and

FIG. 9C is an enlarged view of certain details in FIG. 9B according toembodiments of the present invention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, and components have notbeen described in detail so as not to obscure the present invention.

According to embodiments of the present invention two or more concentricrotors or propellers may be assembled, to operate about a common axis,substantially in a common plane, to provide, each, thrust in the samedirection. Reference is made to FIGS. 2A and 2B, which schematicallydepict a coaxial multi-rotor system and a graph of the rotors' bladesvelocities of this system, respectively, according to embodiments of thepresent invention. Coaxial multi-rotor system 200 may comprise pluralityof rotors 202, 204, 206 and 208 arranged to rotate about a common axis210. The various rotors are assembled to operate substantially in acommon plane in a way that flow through the multi-rotor system will notpass through more than one rotor and the gap between the rotors will beminimal, for example the outer radius of the blades of each rotor isdesigned to be only slightly shorter than the inner radius of the bladesof the neighbor rotor having a longer radius. For example, the outerradius R2 of rotor 204 may be designed to be slightly shorter than theinner radius (not numbered) of rotor 202. This way a substantiallycontinuous coverage of the disk of system 200 is provided by theplurality of rotors, between longest radius R1 and the most inner radiusR5. The plurality of rotors may be rotated in different angular speeds.According to embodiments of the present invention the angular speed ofeach rotor may be that which will rotate the respective rotor blade tipin a tangential V_(TANG MAX) speed which may be the maximal practicaltangential speed for the operation profile of the rotor system, forexample 0.85 of the speed of sound. As depicted in the graph of FIG. 2B,when each of the rotors 202, 204 etc. is set to V_(TANG MAX), thetangential speed of the inner end of the blades of the rotors,V_(TANG INNER(i)), is different, and is slower as the radius Ri (1<i<5)is shorter. It shall be apparent that according to embodiments of thepresent invention the specific rotational speeds of the rotors of system200 may be set otherwise, for example so as to incur rotor's blade tiptangential speed to vary between the rotors, as the functionality of therotor may dictate.

The length of the blades of each of the rotors 202, 204, etc. may bedesigned to achieve desired overall results. For example, the length ofthe rotors' blades may be the same for all of the rotors of system 200,for example to provide lower blades production costs. According to otherembodiments the specific length of blades of each rotor may be designedto achieve optimal aerodynamic efficiency from rotors system 200. Asopposed to a single-rotor system, where a substantial area of the rotoris aerodynamically inefficient, as presented in FIGS. 1A-1C, rotorssystem 200 presents higher utilization of the area occupied by therotor. If, for example, in system 200, R5=0.2R1, the Rotor area(s)Efficiency ratio RE_(S), will be:1−(0.2)²=0.96

Reference is made now to FIG. 3, which schematically presents crosssection through a coaxial multi-rotor system 300, showing only the leftside of the cross section, which is symmetric about symmetry line 300A,according to embodiment of the present invention. Rotors 302, 304, 306,308 and 310 may be substantially arranged in a common plane,perpendicular to the rotation axis, coinciding with line 300A. Each ofthe rotors' blades may have disposed around the outer end of the bladesa rotor end ring 302A-310A, respectively, which may provide structuralsupport as well as aerodynamic air directing means, to minimize therotor's blade-end turbulences and provide improved laminar flow from thespecific rotors of system 300. Providing stator flow guides statorstructures 302B-310B may also reduce the noise produced by the multirotor system by means of reduction of turbulences and other sources ofnoise production such as reduction of zones where flow of air ofdifferent velocities and/or different pressure meet each other.

Reference is made now to FIG. 4, which schematically presents coaxialmulti-rotor system 400, according to embodiments of the presentinvention. Rotors system 400 comprises three coaxial rotors 402, 404 and406 arranged each in a defined plane, the defined planes areperpendicular to the axis of rotation of the rotors. Driving shafts410A, 410B and 410C, mechanically attached to rotors 402, 404, and 406,respectively, and arranged inside each other. The shafts are tubes,except the most internal one, 410C that optionally can be solid rod.Each shaft attached at one side to rotor, and at the other side to motoror generator.

As discussed above, noise caused by the rotation of a rotor or apropeller is mainly due to air shear caused by neighboring/adjacent airflows having different air flow speeds. Therefore, in multi-rotorsystems such as system 300 and system 400, a given total thrust isobtained such that the difference in airspeeds of adjacent airflows islower compared to a single rotor/propeller providing the same thrust andthus the noise caused by that multi-rotor system is lower.

In accordance with embodiments of the present invention, and as ispresented above, due to improved utilization of the rotors disc overallarea, for a required given performance of a rotor (where performance isexpressed by the lift force that may be provided by the rotor) a rotorsystem with outer radius that is substantially shorter than that of asingle rotor with similar performance may be used. This provides furtherbenefits, such as improved maneuvering ability due to quicker responseto control commands (lower inertia) and due to better ability to operateclose to obstacles, as well as operational benefits such as improvedpotential for mobility of the respective aircraft by other vehicles.According to some embodiments of the present invention, a lift providingsystem may comprise two or more coaxial multi-rotor systems. Referenceis made to FIG. 5, which schematically presents lift system 500comprising four coaxial multi-rotor systems 502A-502D, each comprising aplurality of coaxial rotors—three in the example of FIG. 5, according toembodiments of the present invention. Multi-rotors systems 502A-502D maybe disposed for example symmetrically around center point 500A of liftsystem 500. The ratio between the outer radius 502AR of the plurality ofmulti-rotor systems 502A-502D and the outer dimensions A and B of thelift system 500 (in some embodiments A=B) may be designed to meetspecific requirements. For example, multi-rotor systems 502A-502D may bedisposed closer to each other to provide for smaller overall dimensionof the aircraft using lift system 500. In other embodiments multi-rotorsystems 502A-502D may be disposed farther from each other to provide forhigher maneuvering moments. Lift systems such as system 500, havingplurality of multi-rotor systems may also provide higher redundancyfactor, which is a desired benefit since each rotor will be able to workindependently.

A multi-rotor system, such as system 200 or 300, may be disposed within,or equipped with an air guiding system which may comprise one or moreair ducts and/or one or more air guiding fins, which may be static ormoveable to provide changeable guiding angles. Reference is made now toFIG. 6, which is a schematic cross section in coaxial multi-rotor system600, according to embodiments of the present invention. System 600 maycomprise duct 602 comprising multi coaxial rotors 610. Three rotors aredrawn in FIG. 6, however it would be apparent that other number ofcoaxial rotors may be used. Multi-rotor system 610 is disposed withinduct 602, which is adapted to guide the air flowing to and from rotorsystem 610, for example in order to improve the aerodynamic efficiencyof system 600, to reduce the noise produced during the operation ofsystem 600, and the like. Multi-rotor system 610 may be configured toprovide flow which is not parallel to the axis line (non-axial flow) andmay further comprise core element 604 disposed along at least part ofthe central line 600A of system 600. Core 604 may comprise utilitiessuch as rotational driving means (motors, gearboxes and the like),sensors and the like. According to some embodiments system 600 mayfurther comprise air guiding fins 612 disposed adjacent to multi-rotorsystem 610 (down the air flow from the multi-rotor system in the drawnexample). Air guiding fins 612 may be static or may be adapted to changetheir angle with respect to the direction the air flow so as toadaptively address changing conditions such as changes the of airspeedthrough the fins, changes in the turbulences downstream the rotors, etc.Flow lines 620 indicate imaginary lines along duct 602 from the inlettowards its outlet. Internal flow guiding elements may be disposedinside duct 602 to induce, or force improved laminar flow through duct602. Such flow guiding means may be disposed, for example, along, orparallel to the imaginary flow lines.

Reference is made now to FIGS. 7A-7C which are schematic illustrationsof coaxial multi-rotors system 700 in top view, in first cross sectionview and in a second cross section view, respectively, according toembodiments of the present invention. Coaxial multi-rotor system 700comprises two coaxial rotors 701 and 703, disposed substantially in acommon plane wherein the outer radius of inner rotor 703 is slightlyshorter than the inner radius of outer rotor 701. Rotor 701 comprises aplurality of rotating blades 702 and stator fins 702A. Rotor 703comprises a plurality of rotating blades 704 and stator guiding fins704A. Airflow induced through rotors' blades 702 or 704 due to therotation of each of the rotors, passes over the respective guiding fins702A and 704A, respectively. Each of blades 702 and 704 may structurallybe connected to the rotors' circumferential support rings via one ormore structure means 706 and 706A, respectively. In embodiments of theinvention where the angle of attack of the rotors' blades may becontrolled, support means 706 and 706A may provide both structuralsupport and means for changing the angle of attack. Change in the angleof attack of the rotors' blades may be required in order to optimize therotors' performance in changing operation conditions. In someembodiments of the present invention the angle of attack of each of therotors' blades may be within the range of a expressed, for example, indegrees. In a similar manner air guiding fins 702A and 704A maystructurally be supported by at least one support means 708 and 708A,respectively. In embodiments where the angle of attack (or angle of airflow directing) is controllable support means 708 and 708A may also beused for changing the angle of attack of the stator air guiding fins702A and 704A. In some embodiments of the present invention the angle ofattack of each of the stators' guiding fins may be within the range of βdegrees.

Stators disposed downstream of the rotor, as are stators 702A and 704Amay be adapted to straighten helical flow coming out of the rotor andmake it more linear and thus enable extraction of extended thrust fromthat flow. Additionally or alternatively guiding fins such as fins 702Aand/or 704A may be used to deviate the direction of flow and therebychange the direction of thrust.

Each of the rotors, or propellers, in the multi-rotor systems may bepowered, or rotated by means of any known device(s) capable of providingrotational power, such as electrical motor or motors, internalcombustion engine and corresponding gearbox(es), turbo-jet engine andcorresponding gearbox(es), and the like. When silent operation is ofessence electrical motors will be selected, provided that theperformance of the electrical power package—electrical power source andthe electrical motors—will be sufficient. Sufficiency of the electricalpower package may be assessed by the ratio of self-weight to totaloperational weight of the device using this electrical power package,and/or the service time of the electrical power package betweenconsecutive re-charge cycles, and the like.

Since high figure of total operational weight to self-weight of thepower package is of essence Halbach-Array Electrical Motor (HAEM)arrangement may be used to achieve high power with lesser self-weight.Reference is made now to FIGS. 8A-8D, which schematically presentvarious mechanical arrangements 800A-800D respectively, of poweringmulti-rotor systems using electric motors, for example HAEMarrangements, according to embodiments of the present invention. It willbe apparent to those skilled in the art that other powering arrangementsmay be used for powering similar or identical assemblies of rotors. Thevarious powering solutions for providing rotational drive to thecommon-axis, different rotor assemblies are designed for example so thatthe power will be provided to each of the rotors independently toprovide maximum redundancy and in a way that power provided to one rotorwill incur minimal disturbances to the operation of that rotor and theother rotors (e.g. interference with the air flow to/from the rotors,etc.). Each powering unit is designed to provide the required power, therequired rotational speed, the required torque and the requiredcontrollability of those parameters.

Reference is made now to FIG. 8A, which schematically depicts amulti-rotor system 800A shown in schematic half symmetric cross section,according to embodiment of the present invention. System 800A comprisespowering system 820A for providing power to each of the common-axisdifferent rotor length rotor system 810A. Multi-rotor system 800A maycomprise two or more common-axis different-rotor-length rotors 810A1,810A2 and 810A3, arranged one below the other down the flow direction ofair through them rotatable about common axis 802A within air duct 804A.First rotor 810A1 may be suspended by shaft 840A1 which may be theinnermost shaft. Shaft 840A1 may be driven by motor 820A1 which may bepositioned near common axis 802A and lower of motors 820A2 and 820A3 asis described herein below. Second rotor 810A2 may be suspended bysuspension assembly 812A1 to second driving shaft 840A2. Suspensionassembly 812A1 may be formed as a stator for the air flow from firstrotor 810A1, or may be formed as a simple suspension, having minimaldrag, and connecting second rotor 810A2 to its respective driving shaft840A2. Shaft 840A2 may be formed to rotate external to and about innermost shaft 840A1. Second driving shaft 840A2 may be driven by secondmotor 820A2, which may be positioned near common axis 802A and upper ofmotor 820A1 and lower of motor 820A3. Similarly, third rotor 810A3 maybe suspended by suspension assembly 812A2 to third driving shaft 840A3.Suspension assembly 812A2 may be formed as a stator for the air flowfrom first rotor 810A and second rotor 810A2, or may be formed as asimple suspension connecting third rotor 810A3 to its respective drivingshaft 840A3. Shaft 840A3 may be formed to rotate external to and aboutshaft 840A2. Third driving shaft 840A3 may be driven by third motor820A3, which may be positioned near common axis 802A and upper of motors820A1 820A2 with respect to the direction of air flow. As seen in FIG.8A all three motors 820A1-820A3 may be positioned nearby common axis802A and may be suspended by a single suspension assembly 812A3 whichattached to air duct 804A. Suspension assembly 812A3 may be formed,according to embodiments of the invention, as a stator to regulateairflow downstream of rotors 810A1-810A3. According to embodiments ofthe invention two of, or all three motors 820A1-820A3 may be identicalthus providing cheaper and simpler driving solution. Additionally, thedriving solution of FIG. 8A enables use of small diameter bearings whichtypically cheaper and having longer life relative to large diameterbearings. The motors structure of FIG. 8A allow for simpler sealing ofthe motors and shafts, to protect for example against dust and moisture.

In some embodiments more than one duct may be disposed. For example, forrotor 810A1, and/or for rotor 810A2 and/or for rotor 810A3 a separateduct may be provided (not shown in FIG. 8A), thereby regulation of theairflow down stream of rotors 810A1, 810A2 and 810A3 may be improved. Inyet additional embodiments the cross section area of each such duct maybe changed along the axial direction, e.g, the cross section area ofeach duct may be reduced downstream of the flow, or—in anotherembodiment—it may be increased, as the specific intended use maydictate.

Reference is made now to FIG. 8B, which schematically depicts amulti-rotor system 800B shown in schematic half symmetric cross section,according to embodiment of the present invention. System 800B comprisespowering system 820B for providing power to each of the rotors ofcommon-axis different rotor length rotor system 810B. Multi-rotor system800B may comprise two or more common-axis different-rotor-length rotors810B1, 810B2 and 810B3, arranged substantially in a common planeperpendicular to the flow direction of air through them and rotatableabout common Imaginary axis 802B within air duct 804B. First andinner-most rotor 810B1 may be suspended by first bearing 830B1 which maybe installed in the inner portion of air duct 804B. rotor 810B1 may bedriven directly by motor 820B1 which may be, for example, a Halbach typeelectrical motor disposed around the outer circumference of first rotor810B1. The stator portion of first motor 820B1 that typically containelectric coil, may be suspended by suspension bridge 812B which may alsosuspend second bearing 830B2, located at the outer circumference offirst motor 820B1. Second rotor 810B2 may be suspended by second bearing830B2 and may be driven by second motor 820B2. Second motor 820B2 mayalso be, for example, a brushless motor such as for example Halbach typeelectrical motor disposed around the outer circumference of second rotor810B2. The stator portion of second motor 820B2 may be suspended bysuspension bridge 812B which may also suspend third bearing 830B3,located at the outer circumference of second motor 820B2. Third rotor810B3 may be suspended by third bearing 830B3 and may be driven by thirdmotor 820B3. Third motor 820B3 may also be, for example, a Halbach typeelectrical motor disposed around the outer circumference of third rotor810B3. The stator portion of third motor 820B3 may be suspended by theouter portion of air duct 804B. according to the embodiment depicted inFIG. 8B the bearing of each rotor is located at the smaller dimension ofthat rotor while the respective motor is located at the outercircumference of that rotor thus capable of providing higher drivingmoment to its rotor. Additionally, a single and thereby simplersuspension arrangement is required, with minimal interruption by themotors to the air flow and large space left for providing stators ifrequired downstream of the rotors (not shown). The arrangement of FIG.8B additionally provides compact positioning of the rotors, motors andbearings, thus allowing installation in small available spaces.

Reference is made now to FIG. 8C, which schematically depicts amulti-rotor system 800C shown in schematic half symmetric cross section,according to embodiment of the present invention. System 800C comprisespowering system 820C for providing power to each of the rotors ofcommon-axis different rotor length rotor system 810C. Multi-rotor system800C may comprise two or more common-axis different-rotor-length rotors810C1, 810C2 and 810C3, arranged substantially in a common planeperpendicular to the flow direction of air through them and rotatableabout common Imaginary axis 802C within air duct 804C. First andinner-most rotor 810C1 may be suspended by first bearing 830C1 which maybe installed in the inner portion of air duct 804C. Rotor 810C1 may bedriven directly by motor 820C1 which may be, for example, a Halbach typeelectrical motor the rotating portion of which may be disposed adjacentthe outer circumference of first rotor 810C1. The stator portion offirst motor 820C1 suspended by suspension bridge 812C. Suspension bridge812C may extend between the inner portion and the outer portion of airduct 804C, comprising two or more radial elements. Second bearing 830C2may be disposed around the outer circumference of first rotor 810C1.Second rotor 810C2 may be suspended by second bearing 830C2 and may bedriven by second motor 820C2. Second motor 820C2 may also be, forexample, a Halbach type electrical motor the rotating portion of whichmay be disposed adjacent the outer circumference of first rotor 810C2.The stator portion of second motor 820C2 may be suspended by suspensionbridge 812C. Third rotor 810C3 may be suspended by third bearing 830C3and may be driven by third motor 820C3. The inner portion of thirdbearing 830C3 may be disposed around the outer circumference of secondrotor 810C2. Third motor 820C3 may also be, for example, a Halbach typeelectrical motor the rotating portion of which may be disposed adjacentthe outer circumference of third rotor 810C3. The stator portion ofthird motor 820C3 may be suspended by the outer portion of air duct804C. According to the embodiment depicted in FIG. 8C the bearing ofeach rotor is located at the smaller possible dimension of that rotorwhile the respective motor is located at the outer circumference of thatrotor thus capable of providing higher driving moment to its rotor.Additionally, a single and thereby simpler suspension arrangement isrequired, with minimal interruption by the motors to the air flow andlarge space left for providing stators if required downstream of therotors (not shown). The arrangement of FIG. 8C additionally providescompact positioning of the rotors, motors and bearings, thus allowinginstallation in small available spaces. Further, since bearings 830C2and 830C3 are installed at their inner ring, on the outer circumferenceof rotor 810C1 and 810C2, respectively, the actual difference betweenthe rotation speeds of the inner and the outer rings of each of thesebearing is merely the difference in the rotational speeds of therespective pairs of rotors 810C1-810C2 and 810C2-810C3 and thereby aresubject to reduced wear and long life.

Reference is made now to FIG. 8D, which schematically depicts amulti-rotor system 800D shown in schematic half symmetric cross section,according to embodiment of the present invention. System 800D comprisespowering system 820D for providing power to each of the rotors ofcommon-axis different rotor length rotor system 810D. Multi-rotor system800D may comprise two or more common-axis different-rotor-length rotors810D1, 810D2 and 810D3, arranged substantially in a common planeperpendicular to the flow direction of air through them and rotatableabout common Imaginary axis 802D within air duct 804C. Common-axisdifferent rotor length rotor system 810D may be disposed within air duct804D. First and inner-most rotor 810D1 may be suspended, via bearing830D1 directly by the inner portion of air duct 804D. Second rotor 810D2may be suspended onto bearing 830D2 via two or more suspension arms815D2, and bearing 830D2 may be disposed similar and adjacent to bearing830D1 on the inner part of air duct 804D. Third rotor 810D3 may besuspended onto bearing 830D3 via two or more suspension arms 815D3, andbearing 830D3 may be disposed similar and adjacent to bearing 830D1 onthe inner part of air duct 804D. First rotor 810D1 may be drivendirectly by motor 820D1 which may be, for example, a Halbach typeelectrical motor, the inner rotating portion of which may be disposedadjacent the outer circumference of first rotor 810D1. The outerrotating portion of motor 820D1 may be disposed on the innercircumference of second rotor 810D2. Second rotor 810D2 may be drivendirectly by motor 820D2 which may be, for example, a Halbach typeelectrical motor, the inner rotating portion of which may be disposedadjacent the outer circumference of second rotor 810D2. The outerrotating portion of motor 820D2 may be disposed on the innercircumference of second rotor 810D3. Third rotor 810D3 may be drivendirectly by motor 820D3 which may be, for example, a Halbach typeelectrical motor, the inner rotating portion of which may be disposedadjacent the outer circumference of third rotor 810D3. The outer portionof motor 820D3 may be disposed on the inner face of the outer portion ofair duct 804D. According to the embodiment depicted in FIG. 8D thebearing of all of the rotors are located near the central rotation axisthus enabling use of bearings with small diameter which in turn hastechnical advantages (easy to install, to protect and seal and reducedwear) as well as commercial advantages (lower cost). At least some ofthe rotating elements, such as rotors, and suspension arms may be madeof light-weight and strong composite materials such as Carbon reinforcedPolyether ether ketone (PEEK) or epoxy resin based composite materials.Such non-metallic materials have also the benefit of not presentingdisturbances to the magnetic field of the electric motors.

According to embodiments of the invention when low noise produced by amulti-rotor system is of essence electrical motors may be used, asmentioned above with respect to the embodiments depicted in FIGS. 8A-8D.Further, stators and other air guiding means may be used to smooth theair flow to and from the rotors and to minimize turbulences and otherair flow induced noises. In other embodiments other driving means may beused such as internal combustion engine(s), turbo-fan jet engines andthe like. The selection of the driving means may be done to meet thespecific requirements of the multi-rotor system. For example, turbo-fanjet engine driving means may provide improved combination ofthrust-to-weight figure combined with time/service range figure, withrelatively high noise figure while electrical motors may provide silentdriving means with lower thrust-to-net service weight figure and lowertime and range service figure.

According to some embodiments of the present invention multi-rotorsystem may comprise plurality of concentric rotors arranged in more thanone plane or layers. Reference is made to FIGS. 9A and 9B which areschematic cross section of isometric view and front view of the crosssection, respectively, of multi-rotor system 900 according toembodiments of the present invention. Multi-rotor multi-layer system 900comprise at least two concentric multi-rotor units (CMRU) disposedaxially with respect to each other, lower CMRU 902 and upper CMRU 904,arranged about a common rotation axis 900A. Each of CMRU 902 and 904comprise plurality of substantially co-planar and coaxial rotorsdisposed radially with respect to each other, three in the example ofFIGS. 9A and 9B, rotors 902A-902C in the lower CMRU and rotors 904A-904Cin the upper CMRU. Between the outer edges of the blades of each rotorand the inner edges of the blades of the rotor that is external to it,there may be disposed a duct, such as ducts 903A between rotors902A/902B and rotors 904A/904B, respectively, and the like. Ducts903A-903C may be structurally supported by radial structure supports905. Rotors 902A-902C and 904A-904C may be pivotally connected to thestructure of multi-rotor system 900 via circumferential bearings and maybe powered for rotation by electrical motors such as Halbach-ArrayElectrical Motor (HAEM), as explained in details above. Each rotor maybe rotated by a different motor. Each stator of each motor may beattached to a non-rotating structure, for example the duct or the hub901. The duct may therefore provide support for a stator of a HAEM onits inner face and support for bearing on its outer face, as explainedabove, for example with respect to the area of duct 903B marked 950 inFIG. 9B.

Reference is made to FIG. 9C that is schematically showing enlarged viewof details 950 of FIG. 9B, according to embodiments of the presentinvention. On the right (i.e. inner) side of duct 903B a stator of aHAEM may be disposed in area 942 for powering the set of rotatingmagnets on rotor 902B and a stator of a HAEM may be disposed in area 944for powering the set of rotating magnets on rotor 904B. On the left(i.e. outer) side of duct 903B a first (inner) part of a bearing may bedisposed in area 932 for providing pivotal support for rotor 902C and afirst (inner) part of a bearing may be disposed in area 934 forproviding pivotal support for rotor 904C.

According to some embodiments the direction of rotation of each of therotors in each plane may be CW and CCW alternately, as depicted by thearrows in FIG. 9A. Further, the direction of rotation of pairs of lowerand upper rotors may be opposite to each other for maximizing theefficiency of the system.

The arrangement described with respect to FIGS. 9A-9C was found toprovide much higher aerodynamic thrust from a given area of rotorcompared to a single rotor with the same rotor area.

Selection of the internal and external radius of each of the rotors maybe done based on calculations lead by one or more from: substantiallysame provided power to each rotor, common area for all of the rotors,same received thrust from the rotors and substantially same provideddriving moment to each of the rotors.

According to embodiments of the present invention improved redundancymay be achieved with separate rotational power source/driving means foreach rotor and separate power control means for each rotor.Alternatively, one control unit and one driving means may be used todrive a single rotor in each multi-rotor system in the vehicle.

According to embodiments of the present invention a plurality of sensorsmay be provided and disposed with the multi-rotor system. These sensorsmay be adapted to provide indications of one or more of the followingparameters: rotational speed of the rotor, moment provided to the rotor,thrust force produced by each rotor, motors temperature, electric powerentering each motor, air flow speed via the rotors, rotor systemvibrations (magnitude and frequency) in at least one dimension (andpreferably in three linear dimensions and three rotational dimensions)and noise sensor. These physical parameters may be measured or otherwisebe sampled or presented to a control unit or for observation by a userby any known means or methods. For example, the rotational speed androtation direction of the rotor may be measured using measuring meansrelying on the back EMF phenomena. The moment provided to the rotor maybe measured, for example, by measuring the magnitude of axial torque ofa shaft used to provide this moment. The deployment of such sensors maybe maximal, i.e. sensors to measure or reflect all of the measurableparameters of every rotor in every multi-rotor system, or may be lessthan that, as may be dictated by the planned usage.

Noise sensor(s) may be used to evaluate the aerodynamic finiteefficiency of the mechanical moment-to-air thrust conversion state. Itis known that noise stemming from flow turbulences represents, amongother aspects, energy invested in causing turbulences in the fluid (e.g.air). Forming turbulences represents energy invested in non-laminarflow, which is the origin for thrust produced in a fluid environment.Thus, reduction of noise representative of flow turbulences is expectedto increase the mechanical moment-to-air thrust conversion efficiencyfigure. In order to reduce flow noise one or more of the followingvariables may be controlled: rotor rotation speed, rotor blades angle ofattack, stator fins angle of attack, shape of the flow (e.g. air) duct,in case it may be controlled (e.g. half cone (“mouse”) in the air entryto a jet engine, such in various models of the French Dassault Mirageget airplanes, that is used to control the air flow into the jet engineby means of moving it toward the compressor stages of the engine or awayfrom them within the air intake section.

As depicted by the exemplary embodiments of FIGS. 8A-8D and FIGS. 9A-9B,various configurations of the way the rotors in a multi-rotor system areinstalled, suspended, being supported by bearings and finally beingrotated by rotational power sources are possible. The embodiments whereall of the bearings of the rotors are disposed directly on the centralaxis with relatively small radii are beneficial in the sense that thebearing suspension assembly is simple and relatively cheap yet requiresuspension solutions for the outer rotors which provide undesiredincrease in the overall weight and moment of inertia as well as expecteddisturbances to the multi-rotor overall airflow.

Electrical motors disposed to directly drive the respective externalcircumference of the rotor have the benefit of being able to providehigher rotational moments with smaller motors (such as Halbach motor)however such driving solutions may cause complications of mechanicalinstallation of the circumferential motors which typically highlysensitive to changes in the working conditions such as the accuracy ofthe magnetic air gap of the motor, which in such arrangement may besubject to vibrations of the rotor. The Halbach type motor may bedisposed with the magnet array disposed on the rotating rotor and theelectrical winding may be disposed on the multi-rotor static structure.According to embodiments of the invention the motor air gap may beradial or axial, as may fit the specific need and design. The electricmotor used for driving a rotor in the multi-rotor system, whether it isa Halbach type electrical motor, or any other type of electrical motor,may be equipped with electric windings performed using litz wires inorder to increase the electrical efficiency as well as reduce the totalweight of the copper (or other conductive material) required forproviding the electrical power to the motor. Further, in order to enableoperating the electrical motor in extended range of provided power, itmay be cooled by one or more of the following methods: flowing liquidNitrogen between the motor's windings and/or around the magnets; andflowing filtered chilled fluid between the windings and/or around themagnets. When Halbach type electrical motor is used, or any other motorhaving high sensitivity to penetration of contaminants such as dust orsnow to its air gap, means for protecting against such penetration maybe used, such as enhanced mechanical protective sealing, or use ofwipers for removing such contaminants if/when accumulating. Further, inorder to minimize mechanical friction at the rotors' bearings commonmechanical bearings may be replaced by magnetic bearings.

Electrical motors usable for multi-rotor systems according toembodiments of the present invention may be of variety of types as maybe dictated by the selected configuration of the rotor system. As ageneral rule in order to provide maximal mechanical moment suchelectrical motors may use, or may comprise of solutions for reduction ofinefficiency, for example—reduction of the electrical conduits of themotor using high conductivity conduits (super conductor) made of verylow internal resistance materials, operating in super cold environment,etc. It would be apparent to those skilled in the art that certain setof design constrains and requirements imposed on a multi-rotor systemtypically dictate compromised overall solution considering andaddressing the various design requirements as a result of whichtypically none of the requirement is fully answered but the totalperformance is expected to be optimal versus the set of requirements.

According to embodiments of the present invention the thrust produced bya multi-rotor system may be designed so that some of the rotors arerotated in one direction, e.g. clockwise and some are rotated in theother direction, e.g. anticlockwise, thereby the total rotational momentacting on the body of the vehicle may be reduced to minimum or even tozero simply by controlling the total thrust produced by the rotorsturning in one direction and that produced by rotors turning the otherdirection of rotation. The ability to control separately the directionof rotation of each rotor in a multi-rotor system and the thrustprovided by the rotor (e.g. by changing its angle of attack) may beuseful when, for example, one or more rotors in one or more multi-rotorsystems fails to properly operate (e.g. it breaks down or its drivebreaks down). In such cases it may be possible to disconnect the faultyrotor from its drive and compensate for its absence by re-balancing thedirection of rotation and the acquired thrust of each of the remainingrotors.

According to embodiments of the present invention the thrust produced bya multi-rotor system may be decreased in order to decrease the noiseproduced by the multi-rotor system, for example in order to decrease theundesired impact of this noise in an urban area. Decrease of the thrustmay be achieved by one or more of the following means: reduction of therotation speed of one or more of the rotors, adjustment of the rotors'angles of attack so as to decrease the produced noise, adjustment of theangle of the stators, etc.

According to embodiments of the present invention, a multi-rotor systemof the present invention, when used for providing horizontal thrust, maybe manipulated to provide reversing thrust for example for stoppingforward movement or for providing reverse movement with respect to aforward reference movement.

According to embodiments of the present invention one or moremulti-rotor systems of the invention may be used in vehicles capable ofmoving over land, over sea/water pond and/or in air or in anycombination of these environments. Multi-rotor systems of the presentinvention may also be used in manned or in unmanned vehicles.

According to embodiments of the present invention multi-rotor systems ofthe invention may be used for flowing fluids, such as air, gaseousand/or liquids and such fluids which comprise solid particles in them,such as fans, blowers or pumps.

According to embodiments of the present invention rotors in multi-rotorsystems of the invention may be disposed about a common axis so as toprovide thrust in a common axial direction and the rotors may bedisposed shifted axially with respect to each-other.

According to embodiments of the present invention multi-rotor systemsmay be rotated by electrical motor that engages its respective rotor viathe external radius of the rotor or via the internal radius of therotor. Electrical motor engaging the rotor via its external radius mayprovide higher moment with less electrical current.

According to embodiments of the present invention power to energize theelectrical motors may be provided from one or more of the followingelectrical power sources: batteries; fuel cells; fuel that energizes agenerator, solar cells and nuclear reactor.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

What is claimed is:
 1. An airborne vehicle adapted to perform verticaltakeoff and landing (VTOL), comprising: at least two multi-rotorsystems, each multi-rotor system comprising: at least three rotorsrotatable about a common axis, wherein the outer radius of the firstrotor is substantially similar to the inner radius of the second rotor,and the outer radius of a second rotor is substantially similar to theinner radius of the third rotor.
 2. The airborne vehicle of claim 1wherein each of the multi-rotor systems further comprising driving meansadapted to rotate each of the rotors in a rotational speed independentof the rotational speed of the other rotors.
 3. The airborne vehicle ofclaim 1 wherein in at least one of the multi-rotor systems the at leasttwo rotors are disposed and rotatable in a common plane.
 4. The airbornevehicle of claim 1 wherein each of the multi-rotor systems furthercomprising duct disposed closely around the outer radius of the out-mostrotor and directed with its air flow direction coaxially with the commonaxis of the multi-rotor system.
 5. The airborne vehicle of claim 4wherein each of the multi-rotor systems further comprising at least oneadditional air duct disposed closely around outer radius of one otherrotor.
 6. The airborne vehicle of claim 1 wherein each of the rotorscomprises two or more rotor blades evenly spaced around the common axis.7. The airborne vehicle of claim 1 wherein each of the multi-rotorsystems further comprising: at least one stator disposed adjacent to atleast one of the rotors and adapted to control the direction of the airflow that pass through said rotor.
 8. The airborne VTOL vehicle of claim1 adapted to provide mainly vertical thrust when in takeoff and inlanding maneuvering.
 9. The airborne VTOL vehicle of claim 8, furtheradapted to provide mainly horizontal thrust when in flight maneuvering.10. An airborne vehicle adapted to perform vertical takeoff and landing(VTOL), comprising: at least one first multi-rotor system directed toprovide vertical lift, each of the first multi-rotor system comprising:at least three rotors rotatable about a common axis, the outer radius ofthe first rotor is substantially similar to the inner radius of thesecond rotor, and the outer radius of a second rotor is substantiallysimilar to the inner radius of the third rotor; and driving meansadapted to rotate each of the rotors in a rotational speed independentof the rotational speed of the other rotors; and at least one secondmulti-rotor system directed to provide horizontal thrust, each of thesecond multi-rotor system comprising: at least two rotors rotatableabout a common axis, wherein the outer radius of the first rotor issubstantially similar to the inner radius of the second rotor; anddriving means adapted to rotate each of the rotors in a rotational speedindependent of the rotational speed of the other rotors.
 11. Amulti-rotor system for providing air thrust, comprising: at least twomulti-rotor assemblies disposed axially with respect to each other, eachmulti-rotor assembly comprising: at least three rotors rotatable about acommon axis; wherein the outer radius of the first rotor issubstantially similar to the inner radius of the second rotor, and theouter radius of a second rotor is substantially similar to the innerradius of the third rotor.